Protein phosphorylation appears to constitute a major mechanism through which growth factors and related transforming oncogenes influence intracellular events. ln an effort to understand the role of specific protein phosphorylation events ln growth factor mediated cell activation and transformation epidermal growth factor (EGF) was utilized in experiments with several cell lines expressing the EGF receptor tyrosine kinase. Tyrosine kinase activity elicited by EGF. which was manifested as receptor autophosphorylation as well as endogenous substrate phosphorylation. was characterized by both immuroblotting and immunoprecipitation techniques using specific antiphosphotyrosine antibodies. In the EGF-response cells expressing different levels of the EGF receptor protein, preliminary data suggest that EGF causes a dose-dependent increase in receptor tyrosine autophosphorylation which is linear and consistent with a monomeric intramolecular model of EGF receptor activation. Further experiments designed to identify specific tyrosine phosphorylated polypeptide substrate of the EGF receptor kinase were performed using one- and two-dimensional gel electrophoresis coupled with immunoblotting. The results show that several tyrosine phosphoproteins are induced following EGF stimulator. These include proteins of apparent molecular weights of 36, 40, 70, 80 and 150 kd. Similar experiments were carried out using oncogene- transformed cells in an attempt to identify common pathways of mitogenic signal transduction requiring tyrosine phosphorylation. Results suggest an important role of specific protein phosphorylations in cell growth and transformation.